1. Field of the Invention:
The present invention relates to metallizing the face surfaces of thermally stable electrically insulating films of synthetic imide/aromatic polyamide plastics, and also to the intermediate and finished products produced by said metallization. The present invention more especially relates to applying strongly adherent and ductile metallic coatings to flexible, essentially thermosetting polymer films, preferably by electrolytic means, but also by chemical means.
2. Description of the Prior Art:
The pace of the development of electronics in everyday life, as well as in industry, is in part reflected in the number of research projects geared to cost reduction and/or miniaturization of printed circuits and the like.
Flexible films have been extensively used both as a supporting substrate for circuitry (printed circuits) and as various other components in a wide variety of electronic systems. Thus, it is required to provide both the face surfaces and thicknesses thereof with well-defined zones of either electrical conductivity or electrical insulation.
While certain rigid substrates such as paper coated with bakelite and then with phenolic or epoxy resins have long been used to fulfil this need, the present trend is in the direction of flexible substrates of film-forming thermoplastic or thermosetting polymers. These polymers are selected as a function of the intended application and the particular properties required, and include, for example:
Poly(ethylene glycol)terephthalate, polypropylene, polyacetals if high temperature strength is not required, fluoride polymers, polyethersulfones, polyetherimides, polyphenylene sulfides, modified polyphenylene oxides, polyarylates, polybenzimidazoles, polyimide/amides, wholly aromatic polyimides or polyamides, if high strength is desired at elevated temperatures, and the like.
Numerous processes are described in the literature for metallizing the face surfaces of plastic films. The processes which entail lamination by various means to apply a metallic foil onto the plastic film and the processes which entail deposition of conductive or insulating coatings by applying to the base substrate a binder matrix which adheres thereto and which contains conductive or insulating inorganic compounds, are reviewed briefly hereinafter:
First, Process P 25 of Philips in Electronic Production, page 8, December 1973, is pointedly representative of the prior art. It employs a palladium reduction process.
This process features the sensitization of TiO.sub.2 dispersed throughout the volume of the film substrate by light (365 nm radiation) and the reduction of palladium chloride to metallic palladium. It is necessary in a second step to chemically reinforce the metallic surface layer. The process thus has the disadvantages of providing only surface effects and requiring a chemical metallizing step.
Among other processes, that described in U.S. Pat. No. 3,767,538 is also representative. It consists of roughening the surface of a polyester or polyimide film by chemical treatment, of creating conductive palladium nuclei, and of depositing by chemical means or by evaporation a layer of silver, which is subsequently chemically reinforced also. It will thus be seen that such a process is quite lengthy and expensive.
Accordingly, serious need exists in this art for methodology for the fabrication of electronic circuits for industry, the goals of which being:
(i) Substantial reduction in production costs (layout, drilling, stripping, etc.); PA0 (ii) Improvements in the quality of the finished circuits; PA0 (iii) Increased interconnection density; and PA0 (iv) Compliance with the increasingly severe antipollution standards. PA0 (a) First coating the reduced and optionally reinforced conductivised film with a photosensitive resist material; PA0 (b) Developing said photosensitive resist material; PA0 (c) Directly electrolytically reinforcing the resulting film; PA0 (d) Eliminating the developed photosensitive resist material; and PA0 (e) Non-selectively etching away free metal to provide a circuit pattern on said treated film substrate.
It is also required that any such improved methodology provide both rigid and flexible flat circuits and equally metallized holes. A further requirement is good quality of the metallic deposit, e.g., a highly coherent deposit strongly adherent to the support and a ratio of the deposit within the holes to the planar deposit itself being at least as high as 1.